Poor compression of compact flash image, even after 0 fill

Question

Problem: I am attempting to create an exact image of a compact flash and then compress the compact flash image for easier storage. The compact flash is not full (~1.5GB used, 32GB size). The problem is that when I compress the extracted image, it is not compressing well and I am not sure why.

I have tried multiple methods of compression and all result in no better than 50% compression ratio. I would assume that the zeros filling the ~30GB of the CF would compress very well and the resulting file would be ~1.5GB (or better).

What am I doing wrong? What am I overlooking?

Approach

• Zero the CF: dd if=/dev/zero of=/dev/sdb bs=1M
• Partition, Format, install files, data, etc (approx 1.5GB used)
• Create image of CF: dd if=/dev/sdb of=cf.img
• Compress CF (using gzip, bzip2, lzma, etc etc...)

Notes

• I am zeroing the CF by using a USB media reader, connected to a Windows 7 box. The USB device is attached to a virtual machine running Linux.

Answer 1

Answering my own question, but not sure why this is the case.

I tried out dban. Booted the ISO as a virtual machine, and used the "wipe" (write zero) option. Single pass. Followed the same steps afterwards as previously outlined. And the compression worked a LOT better. My compressed image was under 100MB.

The only thing I can guess now is that when using dd, /dev/zero was outputting something funny, or it was not being correctly written to the compact flash.